The present invention relates to thickened cross-linkable polymer compositions which are useful in moulding applications. The invention relates particularly but by no means exclusively to such compositions which are useful for the formulation of sheet moulding compounds, (SMC) and granular moulding compounds (GMC).
SMC is used for a number of applications and generally comprises a leather-like sheet of a cross-linkable polymer composition (also including fillers, chopped glass fibres and other ingredients as necessary) which may be relatively stiff or drapeable to fit a particular mould. The material is then subjected to compression and heating to produce the moulded article. Usually, the base polymer of the composition is an unsaturated polyester with free-COOH groups.
The fundamental requirements for any SMC, of which a typical formulation is shown in Table I below, are:
(1) It must be handleable (i.e. relatively unsticky-tack-free at room temperature so that it can be easily cut to the requirements of a particular mould. PA1 (2) Under prescribed pressures and temperature of moulding all the constituents of the sheet must flow to fill the mould uniformly with no segregation of the components shown in Table 1. PA1 (3) After flowing to the edges of the mould at the prescribed temperatures, the unsaturated resin component must cross-link to give it a permanent form. It should be noted that in the formulation of Table I the unsaturated polyesters cross-link through the styrene present. PA1 (i) The unsaturated resin must be thickened at room temperature to obtain the desired viscosity for handling. PA1 (ii) The viscosity must decrease sharply after placement in the mould to facilitate flow when pressure is applied. PA1 (a) High shear mixing of the particulate fillers and the metal oxides and hydroxides into the resins. PA1 (b) Spreading the glass fibres which are chopped in situ from rovings on to the resultant paste in the form of a sheet moving on a conveyor. PA1 (c) Consolidation of and removal of adventitious air from the resulting fibre reinforced resin sheet; and PA1 (d) Allowing the viscosity of the sheet to increase through slow continuation of the prethickening reaction prior to moulding.
TABLE I ______________________________________ GENERAL PURPOSE SMC FORMULATION Composition/ Ingredients % by weight ______________________________________ *UNSATURATED POLYESTER 25 DISSOLVED IN STYRENE MONOMER *SHRINKAGE CONTROL ADDITIVE 5 *CATALYST 0.3-0.5 *PILLERS (e.g. chalk) 40-50 *RELEASE AGENT 0.4-0.5 THICKENING AGENT 0.2-0.4 GLASS (25 mm) 25 ______________________________________ Ingredients marked * are first mixed together in a high shear mixer
The formulation shown in Table I has an initial viscosity (measured at ambient temperature without the glass reinforcement) of around 200 poise (20 Pa s), while in order to achieve (1) above, industry practice suggests a requirement of around 10000 poise (1 kPa s measured under the same conditions). This viscosity is however too high to permit (2). Thus,, in order to achieve both (1) and (2), two distinct steps are required:
The first step is known as "prethickening" of an SMC, and is based conventionally on the chemical reaction of residual carboxylic acid groups in the unsaturated polyester resin with oxides and hydroxides of Group II metals (typically Magnesium oxide MgO).
The manufacture of an SMC based on the composition shown in Table I consists of four basic steps:
Typically the sheet is stored for some days to allow this maturation to take place. Generally, the sheet reaches the required viscosity after about two days after commencement of the prethickening.
The effect of the chemical reaction is to create a labile network by cross-linking the polyester chains via complex metal salts. The extent of this reaction is dependent upon the level of carboxylic acid groups in the resin and this must be carefully monitored for consistent prethickening behaviour. In practice it is also found that the rate of increase and final extent of viscosity are influenced by both the particle size of the prethickening agent and the level of water in the resin. The increase in viscosity during the mixing stage (a) must not be so great that in stage (b) the fibres are insufficiently wetted by the resin. At the same time conditions and concentrations must be such that maturation is achieved in a reasonable time as indicated above.
A disadvantage of the standard thickening process referred to is that it is not readily reversible. If the prethickened paste is not added sufficiently quickly to the glass fibres in step (b) above, it may be too thick to wet these fibres sufficiently and the whole batch will be lost.
During the moulding of a SMC, the unsaturated monomer reacts in the presence of a catalyst with itself and with the unsaturated bonds of the polymer to form a permanent, covalent network in which the polymer chains are linked through bridges of a few monomer units long. Generally this cross-linking must be effected at a temperature above 100.degree. C. to break down the bonds formed between the Group II metals and the polyester resin.
During this permanent cross-linking reaction the resin shrinks in volume by up to 10%, and unchecked this would not only reduce the fidelity with which the moulding compound reproduced the mould dimensions, but would also render the surface of the moulding compound unattractive by highlighting the presence of the reinforcing fibres.
Hitherto, control of moulding shrinkage polyester-styrene SMCs has had to be effected by adding a solution of a thermoplastic in styrene to the SMC formulation. The solution commonly contains around 30% by weight of the thermoplastic. Appropriate thermoplastics include polystyrene, polyvinyl acetate, polycaprolactone, polymethyl methacrylate, and more recently, certain polybutadienes. Typically, the ratio of unsaturated polyester resin to the solution of thermoplastic is between 90:10 and 60:40 by weight.
An alternative to the use of Group II metal oxides or hydroxides for pre-thickening an SMC formulation is disclosed in GB-A-2111513 (Scott Bader) wherein a crystalline polyester is used as the sole thickening agent. The use of such a polyester has the advantage that no maturation is required and the formulations are ready for use as soon as they have cooled. According to GB-A-2111513 it is preferred that the crystalline polyesters are unsaturated so that they may also take part in the cross-linking reaction with the vinyl monomer (e.g. stryene) during curing. Moreover, it is also preferred that (for ease of handling) the crystalline polyesters are disolved in an aromatic vinyl monomer (e.g. styrene) before being incorporated into the SMC formulation, in which case this monomer also takes part in the cross-linking reaction.
Although the crystalline polyesters disclosed in GB-A-2111531 overcome the need for a long maturation period, it is still necessary to add a thermoplastic resin to reduce or prevent shrinkage during moulding. Furthermore dissolution of the crystalline polyester in an aromatic vinyl monomer represents an additional stage in the process and, moreover, its participation in the cross-linking reaction may undesirably increase the length of the monomer bridges between the polymer chains.
It is an object of the invention to obviate or mitigate the abovementioned disadvantages.